The art of manufacturing organosilane polysulfides is well established, with the art offering a variety of processing strategies.
Meyer-Simon, Schwarze, Thurn, and Michel disclose the reaction of a metal polysulfide with an .OMEGA.-chloroalkyltrialkoxysilane in U.S. Pat. No. 3,842,111. Examples 2 and 3 disclose the preparation of 3,3'-bis (triethoxysilylpropyl) tetrasulfide by reacting Na.sub.2 S.sub.4 with 3-chloropropyltriethoxysilane in water-free alcohol. A procedure for preparing the metal polysulfide is not exemplified.
In U.S. Pat. No. 4,072,701, Pletka describes the preparation of these compounds by first heating 3-chloropropyltrichlorosilane (Example 1) with ethanol, and then adding both sulfur and NaSH. The reaction developed gaseous hydrogen sulfide in situ, but some of the sulfur therein was not recoverable (see, U.S. Pat. No. 4,129,585, col. 1, lines 32-34, in this regard). Therefore, the yields based on added sulfur tended to be low. Also, the use of NaSH is problematic due to its deliquescent nature and its tendency to oxidize to sulfate. The deliquescence is troublesome from the standpoint that it increases the risk that water will enter the reaction and cause hydrolysis of the alkoxide reactants.
After describing the above two patents in U.S. Pat. No. 4,129,585, Buder, Pletka, Michel, Schwarz and D using, describe a procedure for making the noted compounds without the production of gaseous hydrogen sulfide. The process entails reacting a suitable alkali metal alcoholate, e.g., sodium ethoxide, in preferably alcoholic solution with a desired .OMEGA.-chloroalkyltrialkoxysilane and then adding a suitable metal hydrogen sulfide and sulfur. The resulting product was purified by separating the salt formed and distilling off the alcohol. Although the preferred process employed powdered hydrosulfide, the use of the metal hydrogen sulfide can be a source of water entering the system unless precautions are taken. French and Lee also disclosed a process that employed an alkali metal hydrogen sulfide such as NaSH in a separate step (U.S. Pat. No. 5,399,739).
In U.S. Pat. No. 4,507,490, Panster, Michel, Kleinschmidt and Deschler, first prepare Na.sub.2 S. Again, they employ a metal hydrogen sulfide but react it with an alkali metal, such as sodium, in a polar solvent, such as ethanol. This reaction is highly exothermic and evolves hydrogen gas. The process is said to eliminate the use of an alkali metal alcoholate solution, noting that its production requires such a great deal of time as to be industrially improbable. The Na.sub.2 S is reacted with additional sulfur to form a desired polysulfide, preferably Na.sub.2 S.sub.4. The polysulfide is then reacted with a desired .OMEGA.-chloroalkyl trialkoxysilane, e.g., Cl(CH.sub.2).sub.3 Si(OC.sub.2 H.sub.5).sub.3, to form the desired .OMEGA.,.OMEGA.'-bis (trialkoxysilylalkyl) polysulfide.
In U.S. patent application Ser. No. 08/314,204 by Childress (filed Sep. 28, 1994), silane polysulfides are prepared by contacting hydrogen sulfide gas with an active metal alkoxide solution to form a first intermediate product, reacting the first intermediate product with elemental sulfur to form a second intermediate product, and then reacting the second intermediate product with a halohydrocarbylalkoxysilane. In a continuation-in-part, Childress, et al., simplify this process somewhat by combining the first two steps in U.S. patent application Ser. No. 08/383,480 (filed Feb. 2, 1995); hydrogen gas is contacted with an active metal alkoxide and the intermediate product reacted with elemental sulfur and a halohydrocarbylalkoxysilane.
Janssen and Steffen, in U.S. Pat. No. 3,946,059, offer a distinct approach and criticize procedures of the type described above. They eliminate the production, and therefore separation, of salts formed in the above reactions by contacting a bis (alkylalkoxysilyl) disulfide with sulfur at a temperature between 100.degree. and 200.degree. C. This procedure, however, adds the difficulty of the high temperature processing and requires the initial preparation of bis-silyl disulfides by the reaction of sulfuryl chloride with silyl mercaptans. Parker, et al., also disclosed a somewhat complicated process involving a phase transfer catalyst and an aqueous phase (U.S. Pat. No. 5,405,985).
While the possibility might appear to exist that commercial forms or alkali metal sulfides, e.g., sodium tetrasulfide, could be employed, this would not be practical. The commercial forms of sodium tetrasulfide include water which must be completely removed prior to contact with the alkoxylates. If water is present, the alkoxide is hydrolyzed and a polysiloxane polymer is formed. And, while Bittner, et al. disclose in U.S. Pat. No. 4,640,832, the reaction of elemental sodium and sulfur to form polysulfides, this route has certain economic and practical limitations. Reaction temperatures are high; for formation of Na.sub.2 S.sub.3 or Na.sub.2 S.sub.4, for example, the reaction is preferably run at 340.degree. to 360.degree. C. At these temperatures, the polysulfides are corrosive so that choosing an appropriate reactor is imperative; costly alloys of aluminum and magnesium are suggested, as is the use of glassy carbon.
Thus, the prior art has found the generation of hydrogen sulfide gas, the separation of sodium chloride, the preparation of metal alkoxylates, and development of appropriate reaction conditions to be problematic in the preparation of sulfur-containing organosilicon compounds, and did not recognize that there was possible a reaction scheme which efficiently and effectively combines all of them. The invention provides a process which combines these and still obtains high yields based on sulfur.
The prior art also did not recognize that there are numerous possible reaction schemes which avoid the use of either hydrogen sulfide gas or metal hydrogen sulfides, and which provide the desired silane polysulfides efficiently and effectively. These schemes involve process variants with fewer steps than taught by the prior art, with concurrent economic advantages in terms of reduced production times. The invention also provides alternative processes employing these schemes which obtain high yields based on sulfur.